1. Field of the Invention
The present invention relates to a light receiving device with a built-in circuit including a light receiving element (photodiode) for converting light incident thereon into an electric signal and a signal processing circuit, including at least a MOS transistor, for processing a signal output from the light receiving element, the light receiving element and the signal processing circuit being provided on a single substrate. The present invention specifically relates to a light receiving device with a built-in circuit for increasing the response speed of the light receiving element and suppressing malfunction of the MOS transistor.
2. Description of the Related Art
Conventionally, semiconductor devices such as light receiving devices with a built-in circuit, are used in the fields of, for example, optical pickups, optical fibers and photocouplers. Due to the recent increase in the operating speed of CD-ROM, CD-R/RW and DVD-ROM drives and the like, optical pickups now require a high performance light receiving element with a built-in circuit having superior characteristics including high sensitivity, low noise and high response speed. Optical fibers also require a high performance light receiving element with a built-in circuit in order to deal with the increased speed of data transfer.
FIG. 10 shows an exemplary light receiving device with a built-in circuit 900 including a light receiving element and a signal processing circuit provided on a single substrate. The light receiving device with a built-in circuit 900 is described in Japanese Laid-Open Publication No. 11-251567.
The light receiving device with a built-in circuit 900 shown in FIG. 10 includes a P-type semiconductor substrate 30, an N+-type buried diffused layer 31 laminated on the entirety of a surface of the P-type semiconductor substrate 30, and an Nxe2x88x92-type epitaxial layer 32 laminated on the N+-type buried diffused layer 31. The light receiving device with a built-in circuit 900 includes a peripheral circuit 21 as a signal processing circuit and a photodiode 20 as a light receiving element. The peripheral circuit 21 and the photodiode 20 are partially provided in an upper portion of the Nxe2x88x92-type epitaxial layer 32. The peripheral circuit 21 includes MOS transistors 36 and 37, and the photodiode 20 is provide adjacent to the peripheral circuit 21. The photodiode 20 includes, for example, a light receiving area including a P+-type region 33 and N type regions 34, and N+-type diffused regions 35.
The light receiving device with a built-in element 900 having the structure shown in FIG. 10 functions as follows. The N+-type buried diffused layer 31 and N+-type diffused regions 35 together form a potential barrier surrounding the photodiode 20. The potential barrier prevents stray carriers generated in channel regions of the MOS transistors 36 and 37 of the peripheral circuit 21 from entering the photodiode 20, and thus reduces fixed pattern noise (FPN).
The light receiving device with a built-in element 900 having the structure shown in FIG. 10 also functions as follows. Since the N+-type buried diffused layer 31 has a conductivity type which is opposite to the conductivity type of the P-type semiconductor substrate 30 and the photodiode 20 is provided on the N+-type buried diffused layer 31, a P-N junction region is generated at an interface between the P-type semiconductor substrate 30 and the N+-type buried diffused layer 31. The P-N junction region prevents stray carriers generated in the channel regions of the MOS transistors 36 and 37 of the peripheral circuit 21 from entering the photodiode 20, and thus reduces fixed pattern noise.
In developing a light receiving device with a built-in circuit handling signals having a very low amplitude, it is important to prevent stray carriers generated in the MOS transistors 36 and 37 from entering the photodiode 20 and also to prevent stray carriers generated in the photodiode 20 from entering the MOS transistors 36 and 37 and thus generating a wrong signal. Especially in the structure of having the MOS transistors 36 and 37 in the signal processing circuit, an electric current formed of optical carriers generated in the photodiode 20 are likely to flow into the channel regions of the MOS transistors 36 and 37. Therefore, even when the electric current formed of the optical carriers has a very small magnitude, there is an undesirable possibility of the light receiving device malfunctioning.
The light receiving device with a built-in circuit 900 having the above-described structure includes the following problems.
In general, by a usual MOS process, MOS transistors are formed in a P-type semiconductor substrate having a low specific resistance, in order to prevent a latch-up phenomenon which is caused by a parasitic operation between the MOS transistors by stabilizing the entire surface of the P-type semiconductor substrate at the GND potential.
Conversely, the light receiving device with a built-in circuit 900 shown in FIG. 10 includes the N+-type buried diffused layer 31 provided on the entire surface of the P-type semiconductor substrate 30. Therefore, the P-type semiconductor substrate 30, which needs to be stabilized at the GND potential, is electrically separated from the Nxe2x88x92-type epitaxial layer 32 in which the MOS transistors 36 and 37 are formed. The Nxe2x88x92-type epitaxial layer 32 is significantly thinner and thus has a higher specific resistance than the P-type semiconductor substrate 30. Therefore, the Nxe2x88x92-type epitaxial layer 32 has a significantly high resistance in a lateral direction, which is parallel to a surface of the Nxe2x88x92-type epitaxial layer 32. In such a structure, a latch-up phenomenon is very likely to occur. When the latch-up phenomenon occurs, the electric current continues to flow in the chip until the high supply voltage is turned off. As a result, the peripheral circuit 21 does not operate normally. When the electric current continues to flow by the high supply voltage, the temperature of the chip may possibly become abnormally high.
As described above, the N+-type diffused regions 35 provided so as to surround a light receiving region of the photodiode 20 are in contact with the N+-type buried diffused layer 31, and therefore prevent stray carriers generated in the MOS transistors 36 and 37 from entering the photodiode 20. The N+-type diffused regions 35 extend from the surface of the Nxe2x88x92-type epitaxial layer 32 to an interface between the Nxe2x88x92-type epitaxial layer 32 and the N+-type buried diffused layer 31. In order to extend the N+-type diffused regions 35 to the N+-type buried diffused layer 31, the thickness of the N-type epitaxial layer 32 is about 5 xcexcm at most due to the diffusion coefficient of the carriers with respect to the N+-type diffused regions 35. In such a case, a diffusion current component formed by optical carriers which are generated in the vicinity of the P-N junction region at an interface between the P-type semiconductor substrate 30 and the N+-type buried diffused layer 31 exerts the strongest influence on the response speed of the photodiode 20. Since the optical carriers are recombined with the holes by the P-N junction of the P-type semiconductor substrate 30 and the N+-type buried diffused layer 31, the response speed of the photodiode 20 can be increased.
However, optical carriers which are generated by the light incident on the photodiode 20 are mostly generated in a lower portion of the N+-type buried diffused layer 31. Such optical carriers do not contribute to form a photocurrent, which significantly reduces the photoelectric conversion efficiency of the photodiode 20. For example, incident light having a wavelength of 650 nm, which is used in a common optical pickup or the like, penetrates into the photodiode 20 down to a position of a depth of about 4 xcexcm from the surface of the photodiode 20. Therefore, in the case where the Nxe2x88x92-type epitaxial layer 32 has a thickness of 5 xcexcm, about 30% of the incident light does not contribute to form a photocurrent. This significantly reduces an S/N ratio, which represents a characteristic of the photodiode 20 with respect to noise.
In the light receiving device with a built-in circuit 900 shown in FIG. 10, optical carriers which are generated at a position relatively deep in the photodiode 20, or more specifically, in the P-type semiconductor substrate 30 below the N+-type buried diffused layer 31, so astray, migrating towards the MOS transistors 36 and 37. However, such optical carriers are recombined with holes by the P-N junction of the N+-type buried diffused layer 31 and the P-type semiconductor substrate 30 and thus disappear. In this manner, the stray optical carriers are prevented from entering the MOS transistors 36 and 37, and thus the MOS transistors 36 and 37 are prevented from malfunctioning. However, when the optical carriers disappear, the photosensitivity of the photodiode 20 is reduced, resulting in reduction i the S/N ratio thereof. The light receiving device with a built-in circuit 900 also has a problem that a latch-up phenomenon is likely to occur since the Nxe2x88x92-type epitaxial layer 32 has a thickness of as small as 5 xcexcm as compared to the usual thickness of the semiconductor substrate 30 of about 600 xcexcm.
In order to give priority to the response speed and photosensitivity of the photodiode 20, namely, for example, in order to absorb about 90% of light having a wavelength of 650 nm and remove only a diffusion current component, the Nxe2x88x92-type epitaxial layer 32 is required to have a thickness of about 12 xcexcm. Such a structure has a problem that although the photodiode 20 can prevent the stray optical carriers generated in a lower portion of the N+-type buried diffused layer 31 of the photodiode 20 from migrating, but cannot put the N+-type diffused regions 35 into contact with the N+-type buried diffused layer 31. As a result, optical carriers generated in the photodiode 20 flow into the channel regions of the MOS transistors 36 and 37. This increases the undesirable possibility of the MOS transistors 36 and 37 malfunctioning. Even the thickness of the Nxe2x88x92-type epitaxial layer 32 of about 12 xcexcm is not sufficient to suppress generation of the latch-up phenomenon. In addition, heat treatment, which is required to be performed for an extended period of time in order to put the N+-type diffused regions 35 into contact with the N+-type buried diffused layer 31, is not very preferable. The reason for this is because heat treatment performed for an extended period of time excessively diffuses the N+-type diffused regions 35 and thus increases the area of the photodiode 20 as well as the area of the chip.
Japanese Laid-Open Publication No. 3-91959 discloses a structure of using a source diffused region and a drain diffused region of an N-type MOS transistor as the surface regions of a photodiode (corresponding to the P+-type region 33 and N type regions 34 in this example). In this structure, the source diffused region and the drain diffused region of the N-type MOS transistor are used as a cathode electrode of the photodiode, and a P-well diffused region and a P-type buried diffused layer provided below the P-well diffused region are used as an anode electrode of the photodiode. Due to such a structure, the source and drain diffused regions of the N-type MOS transistors can be shallow with a thickness of about 0.2 xcexcm to 0.4 xcexcm, and thus the photodiode 20 can maintain a high level of photosensitivity to light having a short wavelength.
A photodiode having such a structure has a peak photosensitivity at a short wavelength and thus improves the photosensitivity to light having a short wavelength, but has the following problem. The P-type diffused layer and the P-type buried diffused layer have a total thickness of 1.0 xcexcm to 1.5 xcexcm. Due to the potential barrier generated by the P-type buried diffused layer, optical carriers, which are generated at a position deeper than a position in the P-type buried diffused layer having a peak in the impurity concentration, for example, at a position deeper than a position of a depth of 1.5 xcexcm from the surface of the photodiode, do not contribute to form a photocurrent. In this case, the photosensitivity of the photodiode to light having a long wavelength may be significantly reduced. In the case of where, for example, the photodiode uses light having a wavelength of 650 nm (which is used in optical pickups for DVD-ROMs or the like. Optical fiber links, photocouplers or the like), only about 30% of light incident on the photodiode contributes to form a photocurrent. Even when, for example, the thickness of an epitaxial layer is increased to about 3.0 xcexcm in order to avoid deterioration of various characteristics of the MOS transistors and NPN transistors, only 50% of light incident on the photodiode contributes to form a photocurrent in the case where light having a wavelength of 650 nm is used.
In the field of optical pickups used for DVD-ROMs or the like, the wavelength of light used is now being shortened from infrared to red and to blue, in order to increase the data recording density. The structure disclosed in Japanese Laid-Open Publication No. 3-91959 is usable with no practical problem for a special system only for reading light having a short wavelength. By contrast, in the case of pickups for DVD-ROMs or the like, it is necessary to read both light having a short wavelength (such as, for example, blue light) and light having a long wavelength (i.e., red and infrared light). The structure disclosed in Japanese Laid-Open Publication No. 3-91959 may undesirably reduce the photosensitivity to light having a long wavelength and thus significantly deteriorate the S/N ratio.
The structure disclosed in Japanese Laid-Open Publication No. 3-91959 also has the following problem. As described above, the source diffused region and the drain diffused region of the N-type MOS transistor are used as a cathode electrode of the photodiode, and the P-well diffused region and the P-type buried diffused region below the P-well diffused region are used as an anode electrode of the photodiode. Therefore, when the photodiode is supplied with an inverted bias voltage, the depletion layer only expands to about 1.0 xcexcm to 2.0 xcexcm, which is not sufficient. When the depletion layer expands only to such a degree, the junction capacitance of the photodiode increases and thus the response speed of the photodiode decreases. For optical pickups for DVD-ROMs or the like using blue light, the direct current-like photosensitivity to light having a short wavelength is important. However, the low response speed of the photodiode is a serious problem since DVD-ROMs use a frequency band of as high as at least 100 MHz.
A light receiving device with a built-in circuit includes a first conductivity type semiconductor lamination structure; a photodiode for converting light incident thereon to an electric signal by a junction with a first second conductivity type semiconductor layer provided on the first conductivity type semiconductor lamination structure for processing the electric signal obtained by the photoelectric conversion; and a signal processing circuit provided in a region different from the photodiode. The first conductivity type semiconductor lamination structure includes a first conductivity type semiconductor substrate, a first first conductivity type semiconductor layer provided on the first conductivity type semiconductor substrate and having a higher impurity concentration than the first conductivity type semiconductor substrate, and a second first conductivity type semiconductor layer provided on the first first conductivity type semiconductor layer and having a lower impurity concentration than that of the first first conductivity type semiconductor layer. The photodiode is provided in a region surrounded by a third first conductivity type semiconductor layer provided so as to substantially contact a surface of the first first conductivity type semiconductor layer and a fourth first conductivity type semiconductor layer extended from a surface of the first second conductivity type semiconductor layer so as to reach the third first conductivity type semiconductor layer. The signal processing circuit includes at least a MOS structure transistor.
In one embodiment of the invention, the light receiving device with a built-in circuit further includes a fifth first conductivity type semiconductor layer provided below the third first conductivity type semiconductor layer in the state of overlapping at least a portion of the third first conductivity type semiconductor layer, the fifth first conductivity type semiconductor layer running through the second first conductivity type semiconductor layer and reaching at least the first first conductivity type semiconductor layer.
In one embodiment of the invention, the second first conductivity type semiconductor layer has a high specific resistance.
In one embodiment of the invention, the second first conductivity type semiconductor layer has a specific resistance of 200 xcexa9xc2x7cm or more.
In one embodiment of the invention, the light receiving device with a built-in circuit further includes a second second conductivity type semiconductor layer at a surface of the first second conductivity type semiconductor layer.
In one embodiment of the invention, the signal processing section includes an N-type MOS transistor provided so as not to be adjacent to the photodiode.
In one embodiment of the invention, the signal processing section further includes a P-type MOS transistor between the N-type MOS transistor and the photodiode.
In one embodiment of the invention, the signal processing section includes one of the first second conductivity type semiconductor layer and the second second conductivity type semiconductor layer between the N-type MOS transistor and the photodiode. The one of the first second conductivity type semiconductor layer and the second second conductivity type semiconductor layer is set to be at an equal potential to that of the third first conductivity type semiconductor layer.
In one embodiment of the invention, the signal processing section includes one of the first second conductivity type semiconductor layer and the second second conductivity type semiconductor layer between the N-type MOS transistor and the photodiode. The one of the first second conductivity type semiconductor layer and the second second conductivity type semiconductor layer is set to be at a higher potential than that of the third first conductivity type semiconductor layer.
In one embodiment of the invention, the second second conductivity type semiconductor layer is obtained as a result of performing a step of forming a source region and a drain region of the MOS structure transistor.
In one embodiment of the invention, the second second conductivity type semiconductor layer is obtained as a result of diffusion processing performed at least once.
In one embodiment of the invention, the light receiving device with a built-in circuit further includes a sixth first conductivity type semiconductor layer at a surface of the first second conductivity type semiconductor layer.
In one embodiment of the invention, the first second conductivity type semiconductor layer has a substantially uniform impurity concentration in a region below the second second conductivity type semiconductor layer.
In one embodiment of the invention, the first second conductivity type semiconductor layer has a high specific resistance.
In one embodiment of the invention, the first second conductivity type semiconductor layer has a specific resistance of 3.0 xcexa9xc2x7cm or higher.
In one embodiment of the invention, the light receiving device with a built-in circuit further includes a second conductivity type well diffused layer below the second second conductivity type semiconductor layer.
In one embodiment of the invention, the light receiving device with a built-in circuit further includes a first conductivity type well diffused below the sixth first conductivity type semiconductor layer.
In one embodiment of the invention, the second conductivity type well diffused layer is obtained as a result of performing a step of forming a well region of the MOS structure transistor.
In one embodiment of the invention, the first conductivity type well diffused layer is obtained as a result of performing a step of forming a well region of the MOS structure transistor.
In one embodiment of the invention, the light receiving device with a built-in circuit further includes a third second conductivity type semiconductor layer between the first second conductivity type semiconductor layer and the second first conductivity type semiconductor layer.
Thus, the invention described herein makes possible the advantages of providing a light receiving device with a built-in circuit, including a photodiode which has a high level of photosensitivity to light having a short wavelength and is capable of high speed operation, the light receiving device with a built-in circuit preventing optical carriers generated in the photodiode from entering a MOS device and also preventing a latch-up phenomenon.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.